Analytical Contents

The problem of perceptual knowledge is How do we know that the objects we seem to perceive are real? There are four main positions on the nature of perception and perceptual knowledge: (i) direct realism, (ii) indirect realism, (iii) idealism, and (iv) skepticism. I will defend direct realism.


I set forth the strongest arguments for skepticism.

1. The Infinite Regress Argument

A thing cannot be known if there is no justification for believing it. But no belief can be justified, because it would require an infinite regress of reasons.

2. The Problem of the Criterion

There is no noncircular way that we can know our cognitive faculties to be reliable.

3. How Can You Get outside Your Head?

We are only ever directly aware of sensory images, and we can never verify that the sensory images we experience are caused by physical objects.

4. The Brain in a Vat

It is impossible to know that you are not a brain in a vat.

5. Skepticism and Common Sense

Skepticism may be defined as any position that challenges a significant class of common sense beliefs.

6. Skepticism and Internal Justification

Skeptics challenge our justification for believing as we do, in an internalist sense of "justification." Internalist justification involves being able to justify a proposition from the first-person point of view, as opposed to an "external," third-person perspective.


1. Is Skepticism Self-Refuting?

Some forms of skepticism are self-defeating, particularly the view that there is no justification for believing anything. However, the view that there is no justification for beliefs about the external world is not.

2. The G. E. Moore Shift

G. E. Moore argued that if a philosophical theory leads to skepticism, that is a reductio ad absurdum of the theory.

3. Stroud's Defense

Stroud argues that Moore's response fails, that its error is something like begging the question. I show why Stroud is mistaken.

4. Why Study Skepticism?

The nature of knowledge is fundamental to all intellectual endeavor. We must understand the tendencies in our thinking about knowledge that give skeptical arguments their purchase and how to refute skepticism, in order to have an adequate positive understanding of knowledge.


Direct realism holds that in perception, we are directly aware of external phenomena.

1. The Concept of Awareness

Awareness is a relation to an object. It involves a mental state with representational content, an object that corresponds to that content, and a connection between the object and the subject that makes the correspondence nonaccidental.

2. Direct and Indirect Awareness

Direct awareness of a thing is awareness of it which is not based on awareness of anything else. The "based on" relation is defined.

3. The Analysis of Perception

Three conditions are required for one to perceive an object.

3.1. The Existence of Perceptual Experiences

Responding to McDowell and others, I show that perception involves a purely internal mental state, "perceptual experience."

3.2. Content-Satisfaction

In order for one to perceive an object, the object must at least roughly satisfy the content of one's perceptual experience.

3.3. The Causal Condition

In order for one to perceive an object, the object must be causing one's perceptual experience.

4. The Nature of Perceptual Experience

Perceptual experiences typically have the following characteristics.

4.1. Sensory Qualia

The qualitative character of an experience is something over and above its representational content.

4.2. The Representational Content of Experience

Perceptual experiences have representational content that is propositional but nonconceptual.

4.3. The Forcefulness of Perceptual Experience

Forcefulness is the property of perceptual experiences whereby their objects seem to be actually present.

5. Is This Direct Realism?

Perception counts as direct awareness of physical objects, according to my definition of direct awareness in sections IV.1 and IV.2 and the analysis of perception in IV.3. The introduction of perceptual experiences does not make our perception "indirect" in the intended sense.

6. The Mistake of Representationalism 81

Representationalists and idealists confuse a vehicle of awareness with an object of awareness. Locke, Berkeley, and Kant provide examples.


Foundationalism holds that there are propositions we know, that we do not need reasons for, and that all other knowledge rests on these "foundational" propositions.

1. What Is Perceptual Knowledge?

It is knowledge that is directly based on perceptual experiences.

2. Do We Have Perceptual Knowledge?

Knowledge requires justification. Perceptual beliefs are justified because they are supported by perceptual experiences. The experiences themselves are neither justified nor unjustified.

3. A Principle of Foundational Justification

The principle of phenomenal conservatism holds: If it seems to one as if P, then one is thereby at least prima facie justified in believing that P.

4. In Defense of Phenomenal Conservatism

(i) Phenomenal conservatism is self-evident, once one understands the relevant (internalist) notion of justification. (ii) You are already thinking in accordance with phenomenal conservatism, even while trying to evaluate it. (iii) One cannot argue against phenomenal conservatism, because all argument presupposes it.

5. Questions and Objections

I further clarify what phenomenal conservatism entails and defend it against several objections. Among other things, I argue that it is not overly liberal about which beliefs are justified, it does not imply a bad form of relativism, and although it implies that a fallacious argument could render a belief justified, this implication is correct and so does not refute the principle.


1. The Argument from Perspective

The appearance of objects varies depending on the observer's distance and viewing angle. But this does not show that we do not perceive external things. It shows that our awareness is not of intrinsic properties of the object. Instead, it is of objective, relational properties.

2. The Argument from Illusion

The existence of perceptual illusions does not show that we do not perceive real, physical objects. Perceiving an object is compatible with some degree of misperception of its properties.

3. The Argument from Hallucination, Part 1

The existence of hallucinations does not show that we do not perceive real objects when we are not hallucinating. While hallucinating, one does not perceive anything; but normally, one perceives real objects.

4. The Argument from Hallucination, Part 2

Fumerton argues that the possibility of hallucination shows that perceptual beliefs are not justified by virtue of direct acquaintance with objects. I grant this but maintain instead that they are justified by virtue of the forcefulness of perceptual experience.

5. The Argument from Double Vision

Objection: in a case of double vision, you see two of something, but there are not two physical objects; therefore, what you see is something nonphysical. Reply: No, you see a single object, but you see it as being in two different places.

6. The Time-Gap Argument

There is always a delay between the time an event happens and the time of your perceptual experience of it. But this does not show that we do not directly perceive the event, in the relevant sense of "direct."

7. The Causal Argument

There are causal intermediaries between the distal object and one's perceptual experience. But this does not show that we do not directly perceive the distal object, in the relevant sense of "direct."

8. The Illusoriness of Secondary Qualities

Objection: colors do not really exist in the objective, physical world. But since we only see colors and colored objects, it follows that what we see is not physical phenomena. I respond by arguing that colors are in the physical world.


If sense data exist, where are they located? I consider and reject five possible answers.

1. "Sense Data Have No Location"

We may reject this view since the objects of our awareness in perception have shapes, and whatever has shape occupies space.

2. "Sense Data Are in Your Head"

This would be plausible if sense data were brain states; however, our brain states do not have the right spatial and other properties to be the objects of our awareness in perception.

3. "Sense Data Are in the Same Place as the Distal Object"

This view has problems with hallucinations and dreams. It also runs into a conflict with special relativity, for cases of the perception of distant objects.

4. "Sense Data Are Wherever They Appear to Be"

This view encounters problems with fictional locations and indeterminate locations.

5. "Sense Data Are in Phenomenal Space"

This view also conflicts with special relativity. In addition, there are problems with positing causal relationships between items in separate spaces.

6. The Argument from Indeterminacy

A further argument against theories 2-5 arises from the indeterminacy in the content of our perceptual experiences, since no object can actually have indeterminate properties.


On the basis of my theory of perception and perceptual knowledge, I answer the skeptical arguments given in chapter II.

1. The Infinite Regress Argument

The infinite regress is ended when beliefs are based on perceptual experiences. The call for justification does not apply to an experience.

2. The Problem of the Criterion

Gaining knowledge does not require one to know that one's method of gaining knowledge is reliable. That view rests on a confusion between first- and second-order knowledge (knowing and knowing that you know). First-order awareness is directed at the object, not the subject.

3. The Brain in a Vat

The brain-in-a-vat argument turns out to depend upon an indirect realist view of perceptual knowledge, wherein beliefs about the physical world are hypotheses posited to explain the character of our sensory experiences. The brain-in-a-vat scenario is viewed as a competing hypothesis that must be ruled out in order to complete the inference to the best explanation. However, direct realism posits foundational justification for beliefs about the physical world, in accordance with the principle of phenomenal conservatism.

4. Conclusion

We have seen the power of direct realism in accounting for perceptual knowledge and have escaped Hume's disheartening conclusion as to the "perplexity and confusion which is inherent in human nature."